In steam boilers, petrochemical process equipment, feed water heaters, and associated piping, and in various types of pressure vessels in which water is circulated and heat transfer occurs, water insoluble salts deposit on the metallic interior surfaces. The formation or depositation of scale markedly reduces the heat transfer through the walls of such pipes or vessels and furthermore, the capacity of the pipes or vessels and passage of fluids therethrough is restricted by the formation of such scale. Because of the scale formation on the inner surfaces of the pipes, particularly in heat transfer appliances, the pipes are subjected to excessive heat due to the loss of the heat transfer capacity. Further, great pressures are required to overcome the restricting effect of the deposited scale. These disadvantages often lead to leaks and ruptures which necessitate undesirable down time and maintenance costs. Still further, the reduced cross-section of the pipes caused by the formation of the scale can cause an increased pressure drop.
The scale deposit on the vessels usually contains oxides of various metals. Many times vessels are associated with equipment such as condensers, that are constructed of copper alloys, and the scale produced inside the vessel surfaces is frequently found to contain copper scale.
Many different types of methods have been proposed for removing copper scale from a ferrous metal surface. Usually, these involve the use of aqueous acidic solutions containing mineral acids or even organic acids such as citric, acetic, glycolic, or formic.
One method of removing copper scale from a ferrous metal surface involves the use of a strong mineral acid, particularly hydrochloric acid for purposes of dissolving the scale found on the surface of the process equipment, boilers, feed water heaters, and other types of vessels. Although utilization of corrosion inhibitors with mineral acids for cleaning operations has generally prevented acid attack on the equipment surface, such acid treatments have not been universally successful because, when the scale contains copper or copper oxides, many times copper is not dissolved and such copper that is dissolved is redeposited or plated as elemental copper throughout the interior of the vessel surfaces. Such copper depositation serves to accelerate corrosion, particularly in the presence of dissolved ferrous ions, and to interfere with heat exchange reactions.
Another procedure employed for removing copper scale deposits from a ferrous metal surface is to initially contact the surfaces with ammoniacal oxidant wash, such as ammoniacal persulfate, to remove part of the copper deposit followed by contacting the surfaces with a cleaning solution containing an acid plus a copper complexing material. The copper complexing material functions to tie up the copper so that it is dissolved and held in the cleaning solution. Such multistage cleaning procedures however, are expensive to carry out and the copper complexing materials and cleaning solutions which must be used are many times costly. Furthermore, if the deposit containing the copper is thick, long contact times are required to remove the scale which causes additional corrosion of the ferrous metal surfaces.
The present invention provides a method of removing copper deposits from a ferrous metal surface which overcomes or at least mitigates the above-mentioned problems.